This section provides background information related to the present disclosure which is not necessarily prior art.
Certain models of intelligent power strips use power relays, typically rated less than 250V 20 Arms, to switch a line of a receptacle for the main purpose of rebooting a connected load device, e.g., server. Depending upon the load device's internal power supply design, substantial in-rush currents may occur while its input bulk capacitors charge up the moment the relay contacts are closed. This brief, but large current surge, can permanently damage the relay contacts, i.e., weld them close, so they are no longer operative. It can also cause the upstream circuit protection device, typically a circuit breaker, to trip. Some relay manufacturers offer more expensive devices that can handle momentary current surges up to four times their design rating. To further supplement the protection of the relay contacts, the in-rush currents can be mitigated by coordinated timing of relay closure according to the voltage zero-crossing of line frequency.
A number of power strips commonly referred to as Rack PDUs, have switching capabilities associated with all receptacles. PDU stands for power distribution unit and a Rack PDU is used in racks that hold electronic equipment such as servers. The primary reason for the switching capabilities has been two-fold: (a) To be able to remotely recycle power to a connected equipment that is hung up; and (b) to be able to sequentially start up all connected equipment to ensure that upstream breakers do not trip due to all connected loads drawing high in-rush currents concurrently. Typical IT (information technology) loads, for example servers, can draw as much as 5 times their normal current at the time of startup. The above capabilities have typically been addressed in the past through the use of solid state relays at each receptacle.
Bistable relays are increasingly being used in Rack PDU's as they are more energy efficient since their coils do not need to remain energized to maintain the state of their contacts. In such a bistable relay, the coil is pulsed to change the state of the contacts from open to closed and vice-versa. The contacts will then remain in their existing state until the coil is pulsed again. In contrast, in a typical normally open relay, when it is desired to close the contacts of the relay, the coil of the relay must be energized and kept energized to keep the contacts closed. When the coil of the typical normally open relay is de-energized, the relay contacts revert to their normally open state. Similarly, in a typical normally closed relay, when it is desired to open the contacts of the relay, the coil of the relay must be energized and kept energized to keep the contacts open. When the coil of typical normally closed relay is de-energized, the relay contacts revert to their normally closed state. In Rack PDU's having bistable relays, the bistable relays that are closed when there is a loss of power will remain closed. When power is restored, the cumulative in-rush current through the closed bistable relays may cause the upstream circuit protection device, typically a circuit breaker, to trip.